Problem 121 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the coordination compounds (a) \(\mathrm{FeSO}_{4} \cdot\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \cdot 6 \mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{Fe}(\mathrm{CN})_{2} .4 \mathrm{KCN}\) (c) \(\mathrm{KCl} \cdot \mathrm{MgCl}_{2} \cdot 6 \mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{CuSO}_{4} \cdot 4 \mathrm{NH}_{3}\)

Step-by-Step Solution

Answer

Compounds (b) and (d) are coordination compounds.

Step 1: Understand Coordination Compounds

Coordination compounds are complex compounds where a central metal atom is bonded to a surrounding array of molecules or ions, called ligands. They exhibit certain characteristics and contain a coordination complex.

Step 2: Analyze Compound (a)

Compound (a) is \( \mathrm{FeSO}_{4} \cdot\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}\cdot 6 \mathrm{H}_{2} \mathrm{O} \). This compound is known as Mohr's salt. However, there is no coordinate bonding present among the constituents, implying it is not a coordination compound.

Step 3: Analyze Compound (b)

Compound (b) is \( \mathrm{Fe} (\mathrm{CN})_{2} .4 \mathrm{KCN} \). This compound includes \( \mathrm{Fe} (\mathrm{CN})_{6}^{4-} \), which is a coordination complex where \( \mathrm{Fe} \) is bonded to \( \mathrm{CN}^{-} \) ions as ligands. Therefore, it is a coordination compound.

Step 4: Analyze Compound (c)

Compound (c) is \( \mathrm{KCl} \cdot \mathrm{MgCl}_{2} \cdot 6 \mathrm{H}_{2} \mathrm{O} \). It consists of neutral KCl and MgCl₂ with water of crystallization, and there are no coordination bonds, so it is not a coordination compound.

Step 5: Analyze Compound (d)

Compound (d) is \( \mathrm{CuSO}_{4} \cdot 4 \mathrm{NH}_{3} \). This is a known coordination compound, where copper forms a complex with ammonia, represented as \( [\mathrm{Cu} (\mathrm{NH}_{3})_4]^{2+} \). The \( \mathrm{NH}_{3} \) molecules serve as ligands.

Key Concepts

Ligands: The "Claw" of Chemistry

In the world of chemistry, ligands are like friendly claws that grab onto metal atoms to form complexes. These are specific ions or molecules that donate electrons in a process known as donation. Imagine them as helpers that use their electrons to "hold hands" tightly with the central metal in a coordination complex.
Ligands can be:

• Neutral molecules such as water (H₂O) or ammonia (NH₃)
• Anions (negatively charged) such as cyanide (CN⁻) or chloride (Cl⁻)

Each ligand provides at least one pair of electrons to the metal atom, forming what we call a coordinate bond. This unique bond is incredibly important when forming the structure of a coordination compound.
Ligands are essential in defining not only the structure but also the nature and reactivity of coordination compounds. They dictate exactly how the electrons are distributed in the compound, influencing both physical and chemical properties. The beauty of ligands lies in their variability and the diverse roles they play in different complexes.

Coordination Complex: The Heart of the Compound

At the core of a coordination compound, you'll find the coordination complex. This is where the central metal atom or ion is surrounded by a number of ligands. The ligands are positioned in a particular geometric arrangement around the central metal. This can include:

• Octahedral, where six ligands surround the metal
• Tetrahedral, with four ligands forming a pyramid shape
• Square planar, forming a flat plane with four ligands

Each coordination complex has a unique identity based on the type and number of ligands around it. These arrangements determine the properties and behavior of the complex. In the example of compound (b) from the original exercise, \( \mathrm{Fe} (\mathrm{CN})_6^{4-} \), the coordination complex forms due to iron's (Fe) attraction to the six cyanide ions (CN⁻).
The coordination complex is central to understanding how substances interact in chemical reactions. It influences how compounds participate in reactions, impacting factors such as solubility, magnetic properties, and even biological activity.

Coordinate Bonding: The Special Connection

Coordinate bonding is a special type of chemical bond where one atom donates both electrons needed to form a covalent bond. In coordination compounds, this usually involves ligands donating electrons to a metal atom or ion. This bond type is fascinating because it adds a layer of uniqueness to the compound's structure that typical covalent or ionic bonds cannot offer.
For example, in compound (d), \( [\mathrm{Cu} (\mathrm{NH}_3)_4]^{2+} \), ammonia (NH₃) acts as a ligand by donating its electron pair to the copper ion (Cu²⁺), thus forming a coordinate bond.
It's important to remember that while coordinate bonds are primarily characterized by this one-way electron donation, once formed, these bonds behave much like traditional covalent bonds in terms of strength and representation. Understanding coordinate bonding offers insight into the stability and reactivity of coordination compounds, as these unique bonds greatly influence how such compounds will interact in various environments.